Mass-rate liquid flow meters of the recirculation type are disclosed in U.S. Pat. Nos. 3,232,104; 3,232,105; and 3,662,599. In these patents a gear pump is used to recirculate a constant volume flow q. Use of a gear pump is satisfactory for liquids having some lubricating quality, sufficient to keep the gears of the pump from wearing. However, in many applications the liquids being measured have either no lubricity or are chemically corrosive or both. A typical liquid without lubricity is water. Water has a corrosive effect on plain steel gears. Other liquids that have a much more corrosive effect are the many acids and bases that are used in the petrochemical industry. If one were to use a steel gear pump for such liquids, corrosion and wear of the gears would result. This would increase the leakage across the gears and hence change the value of q. One could use gears made out of stainless steel. However, stainless gears present the problem of galling, i.e., the tendency of the gear surfaces to stick or bind when they contact during pump operation. Centrifugal pumps, which have no rubbing surfaces exposed to the liquid flowing through the pump, have previously been considered unsuitable for use in mass-rate flowmeters because the pumping capacity of centrifugal pumps changes considerably with changes in the pressure differential across the pump.
Thus, a centrifugal pump does not have a constant volumetric flow when its pressure rise is varied. Further, centrifugal pump characteristics change with the viscosity of the liquid. Therefore, centrifugal type pumps have not, prior to the present invention, been used for creating the recirculating flow in mass-rate liquid flowmeters such as disclosed in the prior art patents identified above.